0:00:08.597,0:00:13.789
So you just strained a muscle[br]and the inflammation is unbearable.

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You wish you had something [br]ice cold to dull the pain,

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but to use an icepack, you would have had[br]to put it in the freezer hours ago.

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Fortunately, there's another option.

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A cold pack can be left at room temperature[br]until the moment you need it,

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then, just snap it as instructed[br]and within seconds you'll feel the chill.

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But how can something go from [br]room temperature to near freezing

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in such a short time?

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The answer lies in chemistry.

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Your cold pack contains water[br]and a solid compound,

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usually ammonium nitrate, in different[br]compartments separated by a barrier.

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When the barrier is broken,[br]the solid dissolves

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casuing what's known as an [br]endothermic reaction,

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one that absorbs heat from its surroundings.

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To understand how this works,

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we need to look at the two driving[br]forces behind chemical processes:

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energetics and entropy.

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These determine whether a change occurs in[br]a system and how energy flows if it does.

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In chemistry, energetics deals with[br]the attractive and repulsive forces

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between particles at the molecular level.

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This scale is so small, that there are[br]more water molecules in a single glass

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than there are known stars in the universe.

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And all of these trillions [br]of molecules are

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constantly moving, vibrating [br]and rotating at different rates.

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We can think of temperature as[br]a measurement of the average motion,

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or kinetic energy, of all these particles,

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with an increase in movement[br]meaning an increase in temperature,

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and vice versa.

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The flow of heat in any [br]chemical transformation

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depends on the relative strength[br]of particle interactions

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in each of a substance's chemical states.

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When particles have a strong mutual[br]attractive force,

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they move rapidly towards one another,[br]until they get so close,

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that repuslive forces push them away.

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If the initial attraction was [br]strong enough,

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the particles will keep vibrating back[br]and forth in this way.

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The stronger the attraction,[br]the faster their movement,

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and since heat is essentially motion,

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when a substance changes to a state[br]in which these interactions are stronger,

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the system heats up.

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But our cold packs do the opposite,

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which means that when [br]the solid dissolves in the water,

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the new interactions of solid particles [br]and water molecules with each other

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are weaker than the separate interactions[br]that existed before.

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This makes both types of particles [br]slow down on average,

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cooling the whole solution.

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But why would a substance change to a[br]state where the interactions were weaker?

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Wouldn't the stronger preexisting[br]interactions keep the solid from dissolving?

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This is where entropy comes in.

0:02:53.380,0:02:56.271
Entropy basically describes [br]how objects and energy

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are distributed based on random motion.

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If you think of the air in a room, there[br]there are many different possible arrangements

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for the trillions of particles [br]that compose it.

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Some of these will have all [br]the oxygen molecules in one area,

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and all the nitrogen molecules in another.

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But far more will have them [br]mixed together,

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which is why air is always[br]found in this state.

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Now, if there are strong[br]attractive forces between particles,

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the probability of some configurations[br]can change

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even to the point where the odds[br]don't favor certain substances mixing.

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Oil and water not mixing is an example.

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But in the case of the ammonium nitrate,[br]or other substance in your cold pack,

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the attractive forces are not [br]strong enough to change the odds,

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and random motion makes the particles[br]composing the solid separate

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by dissolving into the water[br]and never returning to their solid state.

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To put it simply, your cold pack gets [br]cold because random motion

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creates more configurations where [br]the solid and water mix together

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and all of these have even weaker[br]particle interaction,

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less overall particle movement

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and less heat than there was [br]inside the unused pack.

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So while the disorder that can result[br]from entropy

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may have caused your injury[br]in the first place,

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its also responsible for that[br]comforting cold that soothes your pain.